Optical microscopy is presently the pre-eminent technique for the investigation living cells and tissues. It is known to label biological cells with markers that are stimulated to emit electromagnetic radiation when interrogated with a particular kind of electromagnetic radiation. For example, the markers could be made of fluorescent material or they could be semiconductor nanocrystal quantum dots. In the latter case, see “Multiphoton microscopy based on four-wave mixing of colloidal quantum dots”, Masia et al, Applied Physics Letters 93, 021114 (2008).